One of the main objectives in civil aviation is increasing the lifespan of aircraft.
During the production phase, the assembly of two elements comprising a composite material is always a long and complicated phase, and generally requires the use of a fastener as it is not possible to weld them together, contrary to what was habitually done with metallic elements.
Numerous studies have then aimed at optimising the steps of assembly and process, while maintaining a high level of quality.
The reduction in fuel consumption is also a key factor that has to be taken into account during the design of new aircraft; weight saving can also be achieved, even for a metal structure, by taking into account the assembly stresses in a preliminary design phase, provided that it is possible to subsequently test the assembly.
In order to do this, non-destructive test methods are increasingly preferred, even for more specific purposes such as the assembly phase.
The quality of an assembly is often associated with a stress level reached inside a fastener.
This stress (commonly called prestress or pre-load for the subsequent lifespan when it results from an assembly step) in an assembly comprising elements (in particular metal) has a direct effect on the lifespan of the assembly as regards fatigue. The dimensioning of the elements therefore has an impact on the lifespan of the structure.
It is also possible to achieve weight savings in the design of these items by taking into account these stresses during their dimensioning.
The prestress can be achieved at a final step of the assembly phase when definitive fasteners are used, but also at a step further upstream dedicated to the preparation of the assembly when temporary fasteners are used.
The stress test can be carried out quite easily in a laboratory using a torque-tension test bench, or a piezoelectric wafer incorporated into the assembly, but also by strain measurement of the fasteners, which can be linked to the stress level in the element by a stress-strain behaviour law. Measurement of the strain, and more particularly of the elongation in a chosen direction, can be carried out directly when the structure allows it (for example using a strain gauge or an extensometer), or by using, for example, an ultrasound method.
The determination of stress by a torque-tension test or piezoelectric wafers requires however the use of other elements inside the assembly itself. It is then necessary to use joint components of larger dimension for the test, which then impacts and modifies the rigidity of the assembly, even influencing the performance of the structure, which, as a result, is no longer representative of reality. The interposition of such sensors requires moreover assembly then disassembly of the structure once the measurement has been taken, with use restricted to the laboratory, i.e. it is not possible to incorporate into a definitive structure intended to be used in the finished product.
Furthermore, the methods of measurement by ultrasound are only applicable to certain configurations. These methods are, for example, ineffective when the fasteners are installed with an interference fit which induces inhomogeneous stresses in the element causing disruptions in the measurement of the propagation time of the ultrasonic wave, commonly called “ultrasonic wave time of flight” or when the joining component is crimped, or if a structural element is broken during installation, no longer permitting reference to an initial time of flight of the element before use.
Thus, these techniques are not suitable for the situation in view of the required level of accuracy.
A method involving the use of an optical fibre Bragg grating (FBG) sensor is then advantageously used to evaluate the strains inside an element whatever it is.
The use of a fibre Bragg grating (FBG) positioned in the centre of a fastener is a useful method in that it does not induce disturbance at the level of the functional surfaces of the assembly. Such a measurement is local, and can thus be carried out in a zone with a shallow strain gradient and as a result is indirectly linked to the prestress.
The use of an FBG is for example disclosed in the document U.S. Pat. No. 5,945,665. This document describes a method consisting of drilling a hole at the level of the central core of a fastener, filling the hole with glue (or any other similar material such as for example resin), and installing an optical fibre in it, before polymerization of the glue.
From the point of view of production, installation of the optical fibre is a complicated operation. The glue must be chosen with respect to the material constituting the element, the fibre may be damaged during its installation (in particular with respect to its dimensions, a fibre having a diameter of the order of a few tens of micrometers on average), and it is difficult to obtain accurate positioning of the Bragg grating with respect to the element.
Moreover, when the fastener is then used with an interference fit, this technique can no longer be considered, as the clamping generates compression around the fibre having an effect on the optical response, and no longer allows a reliable analysis of the measurement for determining the strain and then estimating the stress in the fastener.
Other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.